Process for producing a photoelectric conversion device

ABSTRACT

A catalyst element remaining in a first semiconductor film subjected to a first heat treatment (crystallization) is moved and concentrated/collected by subjecting a second semiconductor film which is formed on the first semiconductor film and contains a rare gas element to a second heat treatment. That is, the rare gas element is incorporated into the second semiconductor film to generate a strain field as a gettering site.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a photoelectric conversiondevice wherein a photoelectric conversion layer composed of acrystalline silicon film is formed on a substrate, and a process forproducing the same.

[0003] 2. Related Art

[0004] A photoelectric conversion device can be produced using any oneof various semiconductor materials and organic compound materials.However, the photoelectric conversion device is industrially producedusing silicon mainly. The photoelectric conversion device using siliconcan be classified into a bulk type photoelectric conversion device usinga wafer made of monocrystal silicon or polycrystal silicon and a thinfilm type photoelectric conversion device wherein a silicon film isformed on a substrate. For the bulk type photoelectric conversiondevice, a semiconductor substrate (such as a silicon wafer) is necessaryin the same way as for a LSI (large-scale integrated circuit). Theproduction amount thereof is limited by the supply capacity of thesemiconductor substrate. On the other hand, it is considered thatpotential production capacity of the thin film type photoelectricconversion device is higher because of the use of a semiconductor filmon a given substrate.

[0005] At present, a photoelectric conversion device using amorphoussilicon is made practicable. However, this photoelectric conversiondevice has lower conversion efficiency than the photoelectric conversiondevice using monocrystal silicon or polycrystal silicon. Furthermore,this photoelectric conversion device has problems such as deteriorationby light. Thus, the use of this photoelectric conversion device islimited to products having a small power consumption, such as a pocketcalculator. For sunshine power generation, photoelectric conversiondevices using a silicon film obtained by crystallizing an amorphoussilicon film (the obtained silicon film being referred to as acrystalline silicon film hereinafter) have been actively developed.

[0006] The method of forming the crystalline silicon film is classifiedinto melting recrystallization and solid phase growth methods. In boththe methods, amorphous silicon is formed on a substrate, and thissilicon is recrystallized to form a crystalline silicon film. In eithercase, the substrate is required to endure the crystallizationtemperature of silicon. Thus, the material which can be used for thesubstrate is limited. Particularly in the melting recrystallizationmethod, the material for the substrate is limited to a material enduringthe melting point of silicon, that is, 1412° C.

[0007] The solid phase method is a method of forming an amorphoussilicon film on a substrate and then subjecting the film to heattreatment to crystalline the film. Usually, the amorphous silicon filmis hardly crystallized at a temperature of 500° C. or lower.Practically, it is necessary to heat the amorphous silicon film at 600°C. or higher. For example, in the case that an amorphous silicon filmformed by a vapor growth method is heated to be crystallized, a heatingtime of 10 hours is necessary when heating temperature is 600° C. Whenthe heating temperature is 550° C., a heating time of 100 hours or moreis necessary.

[0008] For the reasons as described above, the substrate for forming acrystalline silicon film is required to have high heat-resistance. It istherefore preferred to use quartz, carbon, a ceramic material or thelike as the material for the substrate. However, such a substrate is notnecessarily suitable for a reduction in production costs. It would beprimarily desired that an inexpensive material circulated in a greatamount in the market is used as the material for the substrate. However,for example, a #7059 glass substrate made by Corning Incorporated, whichis in general frequently used, has a strain point of 593° C. Thus, if aconventional crystallizing technique is used, this substrate isdistorted to generate large deformation. Therefore, the substrate is notused. Since the substrate is made of a material essentially differentfrom silicon, a monocrystal film cannot be obtained even if heattreatment for crystallization is merely performed. As a result, only apolycrystal film can be obtained. The grain size of the polycrystal filmis not easily made large. This fact results in the suppression of animprovement in the efficiency of photoelectric conversion device.

[0009] As a method for solving the above-mentioned problems,JP-A-7-58338 discloses a technique wherein a very small amount of acatalyst element is added as a catalyst material for the promotion ofcrystallization at low temperature, thereby attaining thecrystallization. According to this official gazette open to the public,it becomes possible to make heat treatment temperature low and maketreatment time short. For example, in the case that the heatingtemperature is set to 550° C., it is verified that silicon iscrystallized by heat treatment for 4 hours. The official gazette statesthat a single element of nickel (Ni), iron (Fe), cobalt (Co) or platinum(Pt), a compound of any one of them and silicon, or the like is suitablefor the catalyst element.

[0010] Originally, however, all of the catalyst materials used topromote the crystallization are materials unpreferable for crystallinesilicon. It is therefore desired that the concentration of the catalystmaterial is as low as possible after the crystallization. Theconcentration of the catalyst material necessary for promoting thecrystallization is a range from 1×10¹⁷ to 1×10²⁰/cm³. However, even ifthe concentration is relatively low, the element suitable for thecatalyst material, when taken in silicon, generates a defect levelbecause the element is a metal. Thus, it is evident that this defectlevel causes the deterioration of important characteristics for aphotoelectric conversion device, such as the lifetime of carriers.

[0011] Incidentally, it can be considered that the outline of the actionprinciple of a photoelectric conversion device produced by forming a PNjunction is as follows. The photoelectric conversion device absorbslight, and generates carriers (i.e., electrons and holes) by the energyof the absorbed light. About the generated carriers, the electrons movetoward its n layer and the holes move toward its p layer by drift anddiffusion based on an electric field. In the case that silicon has manydefect levels, the carriers are trapped into the defect levels on theirway to become extinct. That is, the photoelectric conversioncharacteristic of the photoelectric conversion device deteriorates. Thetime from the generation of the electrons and holes to the extinctionthereof is called a lifetime. It is desired that this value is largerfor the photoelectric conversion device. Therefore, it is necessary thatthe amount of impurity elements, which generate the defect level insilicon, are originally as small as possible.

SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide a photoelectricconversion device wherein good use is made of the advantage of thecrystallization of silicon resulting from the above-mentioned catalystmaterial and further the catalyst material which is unnecessary afterthe crystallization is removed to exhibit a superior photoelectricconversion characteristic.

[0013] In order to solve the above-mentioned problems, the process forproducing a photoelectric conversion device of the present inventioncomprises the steps of generating, for a semiconductor film having acrystal structure formed by adding a catalyst element for promotingcrystallization to a semiconductor film having an amorphous structure, astrain field by means of a semiconductor film to which a rare gas isadded or a semiconductor region to which a rare gas is added, as a meansfor removing the catalyst element remaining in the semiconductor filmhaving the crystal structure; and using the strain field as a getteringsite to segregate the catalyst element into this region.

[0014] That is, the process for producing a photoelectric conversiondevice of the present invention comprises the steps of forming a firstsemiconductor film having an amorphous structure; adding a catalystelement for promoting crystallization to the first semiconductor filmhaving the amorphous structure; conducting a first heat treatment toform a first semiconductor film having a crystal structure; forming asecond semiconductor film containing a rare gas element on the firstsemiconductor film having the crystal structure; conducting a secondheat treatment to segregate the catalyst element into the secondsemiconductor film; and removing the second semiconductor film.

[0015] By adding, to the first semiconductor film having the amorphousstructure, the catalyst element for promoting the crystallizationthereof and then subjecting the resultant semiconductor film to thefirst heat treatment, heating temperature necessary for thecrystallization can be made lower than in the prior art. The catalystelement(s) that can be used is/are one or more selected from Fe, Ni, Co,Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.

[0016] The catalyst element remaining in the first semiconductor filmafter the crystallization can be moved into the second semiconductorfilm and concentrated/collected by forming the second semiconductor filmcontaining the rare gas element on the first semiconductor film and thenconducting the second heat treatment. That is, by incorporating the raregas element into the second semiconductor film, a strain field can begenerated so as to be a gettering site. Since the rare gas element isnot basically bonded to another atom, the rare gas element is insertedbetween lattices in the semiconductor film, thereby generating thestrain field.

[0017] The gettering technique is well known as a technique forproducing an integrated circuit using a silicon monocrystal substrate.As the gettering technique, the following are known: extrinsicgettering, wherein a strain field or a chemical effect is supplied to asilicon substrate from the outside so as to generate gettering effect;and intrinsic gettering, wherein a strain field based on lattice defectswith which oxygen generated inside a wafer is concerned is used.Examples of the extrinsic gettering include a method of givingmechanical damage to the back face (that is, the face opposite to theface on which elements are to be formed) of a silicon substrate, and amethod of forming a polycrystal silicon film, and a method of diffusingphosphorus. There is also known a gettering technique performed in thestate that a strain field is generated by secondary lattice defectsformed by ion implantation. The detailed mechanism of the gettering hasnot been necessarily made clear. However, the following phenomenon ispositively used in the mechanism: when heat treatment is conducted asdescribed above, metal elements are precipitated in the region where astrain field is generated.

[0018] In order to remove the second semiconductor film formed on thefirst semiconductor film selectively after the gettering is performed,it is advisable to form a barrier layer on the first semiconductor film.The barrier layer may be formed by treating the first semiconductor filmwith ozone water to form a chemical oxide, by treating the firstsemiconductor film with plasma to oxidize the surface thereof, or byradiating ultraviolet rays in an atmosphere containing oxygen togenerate ozone and oxidizing the surface with ozone.

[0019] The second semiconductor film is formed by sputtering or plasmaCVD. A rare gas element can be taken in the second semiconductor film byincorporating the rare gas into the sputtering gas or adding the raregas to the reaction gas. After the formation of the film, the rare gasmay be added by ion implantation or ion doping. As the rare gas, a gasselected from He, Ne, Ar, Kr and Xe is used.

[0020] The first heat treatment and the second heat treatment areconducted by rapid thermal annealing (RTA) using a halogen lamp, a metalhalide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodiumlamp, or a high-pressure mercury lamp as a heating means or by furnaceannealing. In the present invention, it is sufficient that the getteringcauses the catalyst element to move at a distance correspondingsubstantially to the thickness of the semiconductor film. Thus, thegettering can be completely accomplished even by short-time heattreatment such as RTA.

[0021] According to the present invention, the second semiconductor filmwherein a strain field is generated by the addition of a rare gas isused as a gettering site; therefore, the layer at the incident side oflight in the photoelectric conversion device can be formed as an n-typesemiconductor layer or a p-type semiconductor layer. This makes itpossible to select the substrate on which the semiconductor film havinga crystal structure is formed from various substrates, and select thelayer at the incident side of light freely from both of n-type andp-type semiconductor layers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIGS. 1A to 1D are views which schematically illustrate a processfor producing a photoelectric conversion device of the presentinvention.

[0023]FIGS. 2A to 2D are views which schematically illustrate a processfor producing a photoelectric conversion device of the presentinvention.

[0024]FIG. 3 is a sectional view which illustrates an example of thephotoelectric conversion device of the present invention.

[0025]FIG. 4 is a sectional view which illustrates an example of thephotoelectric conversion device of the present invention.

[0026]FIGS. 5A to 5C are views which schematically illustrate a processfor producing a photoelectric conversion device of the presentinvention.

[0027]FIG. 6 is a graph showing a concentration distribution of argonadded to a crystalline silicon film by ion implantation.

[0028]FIG. 7 is a graph showing concentration distributions of nickeladded to a crystalline silicon film before and after heat treatment.

PREFERRED EMBODIMENTS OF THE INVENTION

[0029] Embodiment 1

[0030] Referring to FIG. 1, the following will describe a process forproducing a photoelectric conversion device by forming a catalystelement for promoting the crystallization of silicon adhesively to anamorphous silicon film, crystallizing the amorphous silicon film by heattreatment, and removing the catalyst element remaining after thecrystallization outside the crystalline silicon film.

[0031] Referring to FIG. 1, a silicon oxide film 102 is formed as anundercoat on a glass substrate (for example, a 1737 glass substrate madeby Corning Incorporated) 101 to have a thickness of 0.3 μm. This siliconoxide film 102 may be formed by plasma CVD using tetramethyl silicate(TEOS) as raw material, or by sputtering. Next, an amorphous siliconfilm 103 is formed by plasma CVD using silane gas as raw material.

[0032] The formation of the amorphous silicon film 103 may be performedusing reduced-pressure thermal CVD, sputtering or vacuum evaporation aswell as plasma CVD. The amorphous silicon film 103 may be an intrinsicamorphous silicon film to which an element of the 13 or 15 group in theperiodic table is not intentionally added, or an amorphous silicon filmto which 0.001 to 0.1 atomic percent of boron (B) is added. Thethickness of the amorphous silicon film 103 is set to 1-20 μm,preferably 5-10 μm (FIG. 1A).

[0033] Next, the workpiece is immersed in an aqueous solution whereinhydrogen peroxide water is mixed with ammonia, and is kept at 70° C. for5 minutes to form an oxidized film (not illustrated) on the surface ofthe amorphous silicon film 103. This oxidized film is formed to improvethe wettability of a solution of nickel acetate in a subsequent step ofapplying the solution. The nickel acetate solution is applied to thesurface of the amorphous silicon film 103 by spin coating. In this way,nickel, which will be a catalyst element, is dispersed onto the surfaceof the amorphous silicon film 103. When the amorphous silicon film 103is crystallized, nickel acts as a catalyst for promoting thecrystallization.

[0034] Next, this workpiece is kept at a temperature of 450° C. in anitrogen atmosphere for 1 hour to cause hydrogen in the amorphoussilicon film 103 to be released. This is performed to lower thethreshold energy for subsequent crystallization by forming danglingbonds (unpaired bonding hands) intentionally in the amorphous siliconfilm 103. Thereafter, the workpiece is subjected to heat treatment at500 to 600° C., preferably at 550° C., in the nitrogen atmosphere for 4to 8 hours, so as to crystallize the amorphous silicon film 103. In thisway, a crystalline silicon film 104 is formed. Because of the catalystaction of nickel, the temperature for this crystallization can be madeto 550° C. This crystalline silicon film 104 contains 0.001 to 5 atomicpercentage of hydrogen. During the above-mentioned heat treatment,nickel diffuses into silicon so as to form a silicide. In this way, thecrystallization of silicon advances.

[0035] As the method of the heat treatment, there is adopted an RTAmethod using a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high-pressure sodium lamp, a high-pressure mercurylamp or the like. In the case of performing the RTA method, a heatinglamp source is lighted for 1 to 60 seconds, preferably 30 to 60 seconds.This is repeated 1 to 10 times, preferably 2 to 6 times. The emissionintensity of the lamp source may be freely decided; however, theintensity is set in such a manner that the semiconductor film isinstantaneously heated up to 600 to 1000° C., preferably 650 to 750° C.Even if such a high temperature is instantaneously realized, thesemiconductor film can be preferentially heated by selecting the kind ofthe lamp source to make the wavelength band of the electromagnetic waveradiated therefrom to an appropriate wavelength band (that is, awavelength band which is sufficiently absorbed in the semiconductorfilm). Accordingly, the substrate 101 itself is not distorted ordeformed. For example, a halogen lamp having a spectrum peak in a bandof wavelengths over 600 nm is suitable for heating the amorphous siliconfilm. By crystallizing the semiconductor film having the amorphousstructure in this way, the crystalline silicon film 104 can be obtainedas well.

[0036] As described above, the crystalline silicon film 104 can beformed on the glass substrate 101. Next, an amorphous silicon film 105containing a rare gas element is formed on the crystalline silicon film104, as illustrated in FIG. 1B. Typically, an amorphous silicon filmcontaining 1×10¹⁹ to 1×10²²/cm³ of argon as the rare gas element isformed by sputtering, so as to have a thickness of 0.05 to 0.1 μm. Inthe sputtering, highly pure silicon and argon (or argon and hydrogen)are used as a target and a sputtering gas, respectively, to form theamorphous silicon film. In order to incorporate a large amount of argoninto the film, it is essential to control the pressure at the time offorming the film. Detailed conditions at this time depend on the usedmachine. For example, by setting the pressure at this time to 0.2 to 1Pa and making film-forming speed relatively slow, the argon content canbe made large.

[0037] Of course, the rare gas element which can be used is not limitedto argon. There may be used helium, krypton, neon or xenon. Theproduction method of the amorphous silicon film 105 is not limited tosputtering. Plasma CVD or vapor deposition may be used if this methodcauses the rare gas element to be incorporated into the silicon film atthe same concentration.

[0038] Thereafter, an electrically heating furnace is used to conductheat treatment at 450 to 800° C., preferably at 550° C., in a nitrogenatmosphere for 1 to 4 hours so that the amorphous silicon film 105containing the rare gas becomes a gettering site. Thus, theconcentration of the catalyst element (nickel) contained in thecrystalline silicon film 104 can be made to 2×10¹⁸cm³ or less. At thistime, the catalyst element moves in the direction shown by an arrow inFIG. 1B, so as to be concentrated/collected in the amorphous siliconfilm 105 containing the rare gas element.

[0039] In the case that an RTA method is used, a heating lamp source islighted for 1 to 60 seconds, preferably 30 to 60 seconds. This isrepeated 1 to 10 times, preferably 2 to 6 times. The emission intensityof the lamp source may be freely decided; however, the intensity is setin such a manner that the semiconductor film is instantaneously heatedup to about 600 to 1000° C., preferably about 700 to 750° C.

[0040] Thereafter, the amorphous semiconductor film 105 is etched to beremoved. This etching is performed by dry etching using NF₃ or CF₄, dryetching without using plasma generated from ClF₃, or wet etching usingan alkali solution such as an aqueous solution containing hydrazine ortetraethylammonium hydroxide (chemical formula: (CH₃)₄NOH).

[0041] By this etching treatment, the surface of the crystalline siliconfilm 104 is exposed. An n-type crystalline silicon film 106 is formed onthis surface. The n-type crystalline silicon film 106 may be formed byplasma CVD or reduced-pressure thermal CVD. It is advisable to form then-type crystalline silicon film 106 so as to have a thickness of 0.02 to0.2 μm, typically a thickness of 0.1 μm. Next, a transparent electrode107 is formed on the n-type crystalline silicon film 106. As thetransparent electrode 107, a film made of an indium tin oxide alloy(ITO) and having a thickness of 0.08 μm is formed by sputtering (FIG.1C).

[0042] Next, lead-out electrodes 108 and 109 are set up. At the time ofsetting the lead-out electrodes 108 and 109, parts of the transparentelectrode 107, the n-type crystalline silicon 105 and the crystallinesilicon 103 are removed, as illustrated in FIG. 1E. Metal films made ofaluminum, silver or the like are formed by sputtering or vacuumevaporation, to fit a positive electrode 108 onto the crystallinesilicon film 104 and fit a negative electrode 109 onto the transparentelectrode 107. The lead-out electrodes 108 and 109 can be formed, usingaluminum, silver, silver paste or the like.

[0043] After the lead-out electrodes 108 and 109 are set up, heattreatment is conducted at 150 to 300° C. for several minutes to improveadhesion between the crystalline silicon film 104 and the silicon oxidefilm 102 as the undercoat. Thus, good electric properties can be gainedSpecifically, an oven is preferably used to conduct heat treatment at200° C. in a nitrogen atmosphere for 30 minutes The above-mentionedprocess makes it possible to yield a photoelectric conversion device.

[0044] Embodiment 2

[0045] According to the process for producing a photoelectric conversiondevice of the present embodiment, in the step of removing the catalystelement for promoting the crystallization of silicon aftercrystallization, a method of using ion implantation or ion doping isadopted to add a rare gas element into the surface of the crystallinesilicon film.

[0046] Referring to FIG. 2, a silicon oxide film 202 is formed as anundercoat on a glass substrate 201 to have a thickness of 0.3 μm. Thissilicon oxide film is formed by plasma CVD using tetramethyl silicate(TEOS) as raw material. This film may be formed by sputtering. Next,silane gas is used as raw material to form an amorphous silicon film 203by plasma CVD. Next, the workpiece is immersed into ammonium hydrogenperoxide, and kept at 70° C. for 5 minutes, thereby forming an oxidefilm (not illustrated) on the surface of the amorphous silicon film 203.A solution of acetate nickel is applied onto the surface of theamorphous silicon film 203 by spin coating. The nickel element functionsas en element for promoting crystallization of the amorphous siliconfilm 203.

[0047] Next, the workpiece is kept at 450° C. in a nitrogen atmospherefor 1 hour to release hydrogen in the amorphous silicon film 203. In thenitrogen atmosphere, the workpiece is subjected to heat treatment at550° C. for 4 to 8 hours, to crystallize the amorphous silicon film 203.In this way, a crystalline silicon film 204 is yielded. The stepsdescribed above are performed in the same way as in the embodiment 1.

[0048] Thereafter, ion doping is used to add a rare gas element to thecrystalline silicon film 204. As the rare gas element, argon is used.The dosage thereof is set to 1×10¹⁴ to 1×10¹⁷/cm², typically 2×10¹⁵/cm²,and acceleration voltage is set to 10 keV. By this doping treatment, aregion containing argon having a concentration of 1×10¹⁸/cm³ or more isformed. This region 205 is formed to extend from the surface of thecrystalline silicon film 204 to points having a depth of about 0.1 μm(FIG. 2B).

[0049] Thereafter, heat treatment is conducted to subject nickelremaining in the crystalline silicon film 204 to gettering. In the caseof using an electrically heating furnace, the heat treatment isconducted at 500 to 800° C., preferably 550° C., in a nitrogenatmosphere for 1 to 4 hours. The heat treatment may be performed by anRTA method. The region 205 into which the rare gas element implanted hasan amorphous structure wherein the crystal is broken. A strain field isgenerated in this region 205 by implanting argon, which has a largeratomic diameter than that of silicon, into the region 205. Thus, theregion 205 can be made to a gettering site. Following the heattreatment, nickel in the crystalline silicon film 204 moves into thisregion 205 so that the concentration of the nickel element in thecrystalline silicon film 204 can be made to 2×10¹⁸/cm³ or less.

[0050] A 100-ppm nickel acetate solution was applied to an amorphoussilicon film having a thickness of 300 nm, and then the film wascrystallized by heat treatment (i.e., annealing) at 550° C. for 4 hoursto yield a crystalline silicon film. A rare gas, argon, was implanted tothe crystalline silicon film by ion doping at an acceleration voltage of10 keV and a dosage of 2×10¹⁵/cm². The distribution of argon obtained atthis time was measured by secondary ion mass spectrometry. The resultsare shown in FIG. 6. Argon was implanted from the surface of thecrystalline silicon film to points having a depth of about 80 nm. Theconcentration thereof was 1×10¹⁸/cm³ or more. FIG. 6 also shows aprofile after the same sample was subjected to heat treatment at 550° C.for 4 hours. The concentration distribution of argon hardly changed. Theargon content in the film did not change. It is therefore understoodthat argon was not released again outside the film at this temperature.The fact that argon is not distributed again by heat treatmentdemonstrates that the gettering site can be stably kept, that is, thatif the argon-added region is removed, no bad effect is produced on thecrystalline silicon film, as will be described later.

[0051]FIG. 7 shows data obtained by measuring a nickel concentrationdistribution in the same sample by secondary ion mass spectrometry. Byheat treatment at 550° C. for 4 hours, the nickel concentration ofnickel in the film was reduced from 5×10¹⁸/cm³ to 1×10¹⁸/cm³. Nickelremoved by the reduction moved to the region to which argon was added(the region from the surface of the crystalline silicon film to pointshaving a depth of about 80 nm). The concentration of nickel in thisregion increased from 1.2×10¹⁹/cm³ to 6×10¹⁹/cm³ as the peak valuethereof. As described above, the data shown in FIG. 7 clearlydemonstrate the effect of the gettering by argon. Of course, such agettering effect can be produced by not only argon but also any otherrare gas elements.

[0052] Furthermore, the region 205 to which argon is added at the highconcentration is not recrystallized by this heat treatment.Consequently, the strain field remains as it is, so as to be a good sitefor gettering.

[0053] For this reason, in order to complete a photoelectric conversiondevice, it is necessary to remove the region 205 by etching or the like.The etching may be performed in the same way as in the embodiment 1.From the crystalline silicon film 204, a region extending from thesurface thereof to points having a depth of about 0.1 μm is removed andsubsequently an n-type crystalline silicon film 206 is formed. The film206 may be formed by plasma CVD or reduced-pressure thermal CVD. Thethickness of the film 206 is preferably set to 0.02 to 0.2 μm, and istypically set to 0.1 μm. Furthermore, a transparent electrode 207 isformed. As the transparent electrode 207, a film made of an indium tinoxide alloy (ITO) and having a thickness of 0.08 μm is formed bysputtering (FIG. 2C).

[0054] In order to form lead-out electrodes 208 and 209, parts of thetransparent electrode 207, the n-type crystalline silicon 206 and thecrystalline silicon 204 are removed, as illustrated in FIG. 2D to exposeparts of the surface of the crystalline silicon film 204. Metal filmsmade of aluminum, silver or the like are formed by sputtering or vacuumevaporation, to fit a positive electrode 208 onto the crystallinesilicon film 204 and fit a negative electrode 209 onto the transparentelectrode 207.

[0055] After the lead-out electrodes 208 and 209 are set up, heattreatment is conducted at 150 to 300° C. for several minutes to improveadhesion between the crystalline silicon film 204 and the silicon oxidefilm 202 as the undercoat. Thus, good electric properties can be gained.In the present embodiment, an oven is used to conduct heat treatment at200° C. in a nitrogen atmosphere for 30 minutes. The above-mentionedprocess makes it possible to yield a photoelectric conversion device.

[0056] Embodiment 3

[0057] The following will describe an embodiment wherein an amorphoussilicon film formed as a gettering site on a crystalline silicon filmformed using a catalyst element is selectively removed.

[0058] Referring to FIG. 5A, a silicon oxide film 502 is formed as anundercoat on a glass substrate 501 in the same way as in the embodiment1, so as to have a thickness of 0.3 μm. Next, an amorphous silicon filmis made from silane gas as raw material by plasma CVD, and then acatalyst element is introduced thereto so as to crystallize theamorphous silicon film. In this way, a crystalline silicon film 503 isformed. The steps described above are performed in the same way as inthe embodiment 1.

[0059] Next, a barrier layer 504 is formed on the surface of thecrystalline silicon film 503. To form the barrier layer, a chemicaloxide produced by treatment with ozone water may be used. The chemicaloxide may also be produced by treatment with an aqueous solution whereinhydrogen peroxide water is mixed with sulfuric acid, hydrochloric acid,nitric acid or the like. In a different method, the chemical oxide maybe produced by plasma treatment in an oxidizing atmosphere or oxidizingtreatment with ozone generated by radiation of ultraviolet rays in anoxygen-containing atmosphere. Alternatively, the crystalline siliconfilm may be heated at 200 to 350° C. in a clean oven to form a thinoxidized film as the barrier layer, or an oxidized film having athickness of 1 to 5 nm is deposited as the barrier layer by plasma CVD,sputtering or vapor deposition.

[0060] Thereafter, an amorphous silicon film 505 containing a rare gaselement is formed on the barrier layer 504. This film may be formed inthe same way as in the embodiment 1. Alternatively, after the formationof an amorphous silicon film, the rare gas element may be added theretoby ion implantation or ion doping, as described in the above-mentioneditem [Embodiment 2]. This amorphous silicon film 505 is used as agettering site.

[0061] In order to subject the catalyst element remaining in thecrystalline silicon film 503 to gettering, heat treatment is conducted.The heat treatment is conducted at 500 to 800° C., preferably 550° C.,in a nitrogen atmosphere for 1 to 4 hours. In the case that an RTAmethod is used, a heating lamp source is lighted for 1 to 60 seconds,preferably 30 to 60 seconds. This is repeated 1 to 10 times, preferably2 to 6 times. The emission intensity of the lamp source may be freelydecided; however, the intensity is set in such a manner that thesemiconductor film is instantaneously heated up to about 600 to 1000°C., preferably about 700 to 750° C.

[0062] Thereafter, the amorphous silicone film 505 is selectively etchedto be removed. This etching is performed by dry etching without the useof plasma generated from ClF₃, or wet etching using an alkali solutionsuch as an aqueous solution containing hydrazine or tetraethylammoniumhydroxide (chemical formula: (CH₃)₄NOH). At this time, the barrier layer504 functions as an etching stopper. Thereafter, the barrier layer 504should be removed with hydrofluoric acid.

[0063] In this way, the catalyst element in the crystalline silicon film503 is subjected to gettering into the amorphous silicon film 505 towhich the rare gas element is added, so as to make it possible to setthe concentration of the catalyst element in the crystalline siliconfilm 503 to 2×10¹⁸/cm³ or less.

[0064] An n-type crystalline silicon film 506 is formed on the surfacethereof. This film 506 may be formed by plasma CVD or reduced-pressurethermal CVD. The thickness of the film 506 is set to 0.02 to 0.2 μm.Next, as a transparent electrode 507, a film made of an indium tin oxidealloy (ITO) and having a thickness of 0.08 μm is formed on the n-typecrystalline silicon film 506 by sputtering (FIG. 5B).

[0065] Furthermore, lead-out electrodes 508 and 509 are set up. At thetime of setting the lead-out electrodes 508 and 509, parts of thetransparent electrode 507, the n-type crystalline silicon 506 and thecrystalline silicon film 503 are removed, as illustrated in FIG. 5C.Metal films made of aluminum, silver or the like are formed bysputtering or vacuum evaporation, to fit a positive electrode 508 ontothe crystalline silicon film 503 and fit a negative electrode 509 ontothe transparent electrode 507. The lead-out electrodes 508 and 509 canbe formed, using aluminum, silver, silver paste or the like.

[0066] After the lead-out electrodes 508 and 509 are set up, heattreatment is conducted at 150 to 300° C. for several minutes to improveadhesion between the crystalline silicon film 503 and the silicon oxidefilm 502 as the undercoat. Thus, good electric properties can be gained.In the present embodiment, an oven is used to conduct heat treatment at200° C. in a nitrogen atmosphere for 30 minutes. The above-mentionedprocess makes it possible to complete a photoelectric conversion device.The present embodiment may be combined with the embodiment 1 orembodiment 2.

[0067] Embodiment 4

[0068] The following will describe, as the present embodiment, anembodiment wherein the surface of the crystalline silicon film issubjected to anisotropic etching in the process for producing thephotoelectric conversion device described as the embodiment 1, 2 or 3,so as to make one layer in the photoelectric conversion device uneven.The technique where in the surface is made uneven to reduce the surfacereflection of the photoelectric conversion device is called texturetechnique.

[0069] A silicon oxide film is formed as an undercoat 302 on a glasssubstrate 301 (for example, a Corning 7959 glass substrate) to have athickness of 0.3 μm. This silicon oxide film is formed by plasma CVDusing tetramethyl silicate (TEOS) as raw material. This film may beformed by sputtering. Next, an amorphous silicon film is formed byplasma CVD. This amorphous silicon film may be formed byreduced-pressure thermal CVD, sputtering or vacuum evaporation as wellas plasma CVD. The amorphous silicon film may be a substantiallyintrinsic amorphous silicon film, or an amorphous silicon film to which0.001 to 0.1 atomic percent of boron (B) is added. The thickness of theamorphous silicon film 103 is set to 20 μm. Of course, this thicknessmay be changed to a desired thickness.

[0070] The workpiece is subjected to heat treatment to form acrystalline silicon film 303. In the crystalline silicon film 303, theconcentration of the catalyst element introduced in the step of thecrystallization is reduced to 2×10¹⁸/cm³ or less by the getteringtreatment of the present invention.

[0071] After the gettering treatment, the surface of the crystallinesilicon film 303 is subjected to a texture treatment. The texturetreatment can be conducted using an aqueous solution of sodiumhydroxide, or hydrazine. The following will describe a case in whichsodium hydroxide is used.

[0072] The texture treatment is conducted by heating an aqueous solutionhaving a sodium hydroxide concentration of 2% to 80° C. Under thiscondition, the etching rate of the crystalline silicon film 303 used inthe present embodiment can be about 1 μm/minute. The etching isperformed for 5 minutes. Thereafter, the workpiece is immersed intoboiling water in order to stop the reaction instantaneously.Furthermore, the workpiece is sufficiently washed with flowing water.When the surface of the crystalline silicon film 303 after the texturetreatment is observed with an electron microscope, irregularities, whichare arranged at random and have a height of about 1 to 5 μm, can beobserved.

[0073] An n-type crystalline silicon film 304 is formed on this surface.The n-type crystalline silicon film 304 may be formed by plasma CVD orreduced-pressure CVD. The thickness of the n-type crystalline siliconfilm 304 is preferably set to 0.02 to 0.2 μm. In the present embodiment,the thickness is set to 0.1 μm.

[0074] Next, a transparent electrode 305 is formed on the n-typecrystalline silicon film 304. As the transparent electrode 305, a filmmade of an indium tin oxide alloy (ITO) and having a thickness of 0.08μm is formed by sputtering. Finally, lead-out electrodes are fitted up.In order to fit up the lead-out electrodes, parts of the transparentelectrode, n-type crystalline silicon and the crystalline silicon film303 are removed and subsequently a negative electrode 306 is set up onthe transparent electrode 304 and a positive electrode 306 is set up onthe crystalline silicon film 303, as illustrated in FIG. 3. The lead-outelectrodes 306 may be formed by sputtering or vacuum evaporation usingaluminum, silver, silver paste or the like. After the electrodes 306 arefitted up, the workpiece is subjected to heat treatment at 150 to 300°C. for several minutes to improve adhesion between the crystallinesilicon film 303 and the under coat 302. Thus, good electric propertiescan be gained. In the present embodiment, an oven is used to conductheat treatment at 200° C. in a nitrogen atmosphere for 30 minutes. Theabove-mentioned process makes it possible to yield a photoelectricconversion device having, in the surface thereof, a texture structure.The present embodiment may be freely combined with the embodiment 1, 2or 3.

[0075] Embodiment 5

[0076] The following will describe, as the present embodiment, atechnique of forming a coating film made of a catalyst element forpromoting the crystallization of silicon, forming an amorphous siliconfilm closely onto the coating film made of the catalyst element,crystallizing the amorphous silicon film by heat treatment, andsubsequently removing the catalyst element diffused into the resultantcrystalline silicon film to produce a photoelectric conversion device,referring to FIG. 4.

[0077] First, a coating film made of a catalyst element for promotingthe crystallization of silicon is formed on a substrate. As the catalystelement, nickel is typically used. A silicon oxide film having athickness of 0.3 μm is firstly formed as an undercoat 402 on a glasssubstrate (for example, a Corning 7059 glass substrate) 401. Thissilicon oxide film is formed by plasma CVD using tetramethyl silicate(TEOS) as raw material. This film may be formed by sputtering. Next, anickel film 407 is formed on this substrate. The nickel film is formedby electron beam vacuum evaporation using a tablet made of pure nickel,so as to have a thickness of 0.1 μm. Next, an amorphous silicon film isformed by plasma CVD. The formation of the amorphous silicon film may beperformed using reduced-pressure thermal CVD, sputtering or vacuumevaporation as well as plasma CVD. The amorphous silicon film 103 may bea pure amorphous silicon film to which no element of the 13 or 15 groupin the periodic table is added, or an amorphous silicon film to which0.001 to 0.1 atomic percent of boron (B) is added. The thickness of theamorphous silicon film is set to 10 μm. Of course, this thickness may bechanged to a desired thickness.

[0078] Next, the workpiece is kept at a temperature of 450° C. in anitrogen atmosphere for 1 hour to release hydrogen in the amorphoussilicon film. This is performed to lower the threshold energy forsubsequent crystallization by forming dangling bonds (unpaired bondinghands) intentionally in the amorphous silicon film. Thereafter, theworkpiece is subjected to heat treatment at 550° C. in the nitrogenatmosphere for 4 to 8 hours, so as to crystallize the amorphous siliconfilm. In this way, a crystalline silicon film 403 is yielded. Because ofthe catalyst action of nickel, the temperature for this crystallizationcan be made to 550° C. This resultant crystalline silicon film 403contains 0.001 to 5 atomic percentage of hydrogen. The nickel elementdiffuses partially from the nickel film 407 to the silicon film, so thatthe crystallization advances. In this way, the crystalline silicon filmcan be formed over the glass substrate.

[0079] Thereafter, a gettering site is formed to perform gettering. Thegettering site may be composed of an amorphous silicon film containing arare gas element, or a region to which a rare gas is implanted insidethe crystalline silicon film 403. Thereafter, the workpiece is subjectedto heat treatment to perform gettering. The gettering site which hasbeen unnecessary is removed by etching.

[0080] In this way, the surface of the crystalline silicon film 403 isexposed over the main surface of the substrate 401. An n-typecrystalline silicon film 404 is formed on this surface. Next, as atransparent electrode 405, a film made of an indium tin oxide alloy(ITO) and having a thickness of 0.08 μm is formed on the n-typecrystalline silicon film 404 by sputtering. Finally, lead-out electrodes406 are fitted up to complete a photoelectric conversion device. Theembodiment of the present embodiment may be freely combined with theembodiment 1, 2, 3 or 4.

[0081] Embodiment 6

[0082] Referring to FIG. 1, the following will describe, as the presentembodiment, a photoelectric conversion device wherein a p-typesemiconductor layer is positioned at the incident side of light, whichis different from the embodiment 1.

[0083] Referring to FIG. 1, a silicon oxide film 102 as an undercoat andan amorphous silicon film 103 are firstly formed on a stainless steelsubstrate 101. The amorphous silicon film 103 may be an intrinsicamorphous silicon film to which an element of the 13 or 15 group in theperiodic table is not intentionally added, or an amorphous silicon filmto which 0.001 to 0.1 atomic percent of phosphorus (P) is added. Thethickness of the amorphous silicon film 103 is set to 1-20 μm,preferably 5-10 μm (FIG. 1A).

[0084] Next, an oxidized film (not illustrated) is formed on the surfaceof the amorphous silicon film 103, and a nickel acetate solution isapplied onto the surface of the amorphous silicon film 103 by spincoating, so as to disperse nickel, which will be a catalyst element,into the surface of the amorphous silicon film 103.

[0085] Next, the workpiece is kept at a temperature of 450° C. in anitrogen atmosphere for 1 hour, to release hydrogen in the amorphoussilicon film 103. Thereafter, the workpiece is subjected to heattreatment at 500 to 600° C., preferably at 550° C., in the nitrogenatmosphere for 4 to 8 hours, to crystallize the amorphous silicon film103. In this way, a crystalline silicon film 104 is formed.

[0086] As described above, the crystalline silicon film 104 can beformed on the stainless steel substrate 101. Next, an amorphous siliconfilm 105 containing a rare gas element is formed on the amorphoussilicon film 104, as illustrated in FIG. 1B.

[0087] Thereafter, an electrically heating furnace is used to conductheat treatment at 450 to 800° C., preferably at 550° C., in a nitrogenatmosphere for 1 to 4 hours so that the amorphous silicon film 105containing the rare gas element becomes a gettering site. Thus, theconcentration of the catalyst element (nickel) contained in thecrystalline silicon film 104 can be made to 2 ×10¹⁸cm³ or less. At thistime, the catalyst element moves in the direction shown by an arrow inFIG. 1B, so as to be concentrated/collected in the amorphous siliconfilm 105 containing the rare gas element.

[0088] Thereafter, the amorphous semiconductor film 105 is etched to beremoved. This etching is performed by dry etching using NF₃ or CF₄, dryetching without the use of plasma generated from ClF₃, or wet etchingusing an alkali solution such as an aqueous solution containinghydrazine or tetraethylammonium hydroxide (chemical formula: (CH₃)₄NOH).

[0089] By this etching treatment, the surface of the crystalline siliconfilm 104 is exposed. A p-type crystalline silicon film 106 is formed onthis surface. The p-type crystalline silicon film 106 may be formed byplasma CVD or reduced-pressure thermal CVD. It is advisable to form thep-type crystalline silicon film 106 so as to have a thickness of 0.02 to0.2 μm, typically a thickness of 0.1 μm. Next, a transparent electrode107 is formed on the p-type crystalline silicon film 106. As thetransparent electrode 107, a film made of an indium tin oxide alloy(ITO) and having a thickness of 0.08 μm is formed by sputtering (FIG.1C).

[0090] Finally, lead-out electrodes 108 and 109 are fitted up. Ifnecessary, the workpiece is subjected to heat treatment at 150 to 300°C. for several minutes, so as to complete a photoelectric conversiondevice wherein light is radiated onto the side of the p-type crystallinesilicon film.

[0091] Embodiment 7

[0092] If a p-type crystalline silicon film is formed instead of then-type crystalline silicon film 206 in the embodiment 2 described withreference to FIG. 2, a photoelectric conversion device wherein light isradiated onto the side of the p-type crystalline silicon film can beproduced.

[0093] In the process for producing a photoelectric conversion deviceaccording to the present invention, a crystalline silicon film can beobtained at a lower heat treatment temperature than in the prior art byusing a catalyst material such as nickel in the step of subjecting anamorphous silicon film to heat treatment so as to be crystallized.Additionally, the concentration of the catalyst material remaining inthe resultant crystalline silicon film can be reduced. As a result, itis possible to obtain a photoelectric conversion device using aninexpensive glass substrate and exhibiting a superior photoelectricconversion characteristic.

[0094] Furthermore, a second semiconductor film to which a rare gaselement is added so as to generate a strain field is used as a getteringsite. In the photoelectric conversion device, therefore, its layer atthe incident side of light can be made to an n-type semiconductor layer,or can be made to a p-type semiconductor layer. Thus, even if anontransparent substrate such as a stainless steel substrate is used,the layer at the light-incident side can be freely selected from both ofn-type and p-type semiconductor layers.

What is claimed is:
 1. A process for producing a photoelectricconversion device, comprising the steps of: forming a firstsemiconductor film having an amorphous structure; adding an element forpromoting crystallization to the semiconductor film having the amorphousstructure; conducting a first heat treatment to form a firstsemiconductor film having a crystal structure; forming a secondsemiconductor film containing a rare gas element over the firstsemiconductor film having the crystal structure; conducting a secondheat treatment to segregate the element for promoting crystallizationinto the second semiconductor film; and removing the secondsemiconductor film.
 2. A process according to claim 1, wherein thesecond semiconductor film is formed by sputtering.
 3. A processaccording to claim 1, wherein the second semiconductor film is formed byplasma CVD.
 4. A process according to claim 1, wherein the first heattreatment is conducted by radiation from one or more selected from ahalogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp,a high-pressure sodium lamp, or a high-pressure mercury lamp.
 5. Aprocess according to claim 1, wherein the first heat treatment isconducted by furnace annealing using an electrically heating furnace. 6.A process according to claim 1, wherein the second heat treatment isconducted by radiation from one or more selected from a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp.
 7. A process according toclaim 1, wherein the second heat treatment is conducted by furnaceannealing using an electrically heating furnace.
 8. A process accordingto claim 1, wherein the rare gas is one or more selected from He, Ne,Ar, Kr and Xe.
 9. A process according to claim 1, wherein the catalystelement is one or more selected from Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt,Cu and Au.
 10. A process for producing a photoelectric conversiondevice, comprising the steps of: forming a first semiconductor filmhaving an amorphous structure; adding an element for promotingcrystallization to the semiconductor film having the amorphousstructure; conducting a first heat treatment to form a firstsemiconductor film having a crystal structure; forming a barrier layerover a surface of the first semiconductor film having the amorphousstructure; forming a second semiconductor film containing a rare gaselement over the barrier layer, conducting a second heat treatment tosegregate the element for promoting crystallization into the secondsemiconductor film, and removing the second semiconductor film.
 11. Aprocess according to claim 10, wherein the second semiconductor film isformed by sputtering.
 12. A process according to claim 10, wherein thesecond semiconductor film is formed by plasma CVD.
 13. A processaccording to claim 10, wherein the barrier layer comprises a chemicaloxide film formed using ozone water.
 14. A process according to claim10, wherein the barrier layer is formed by oxidizing the surface of thefirst semiconductor film having the amorphous structure by plasmatreatment.
 15. A process according to claim 10, wherein the barrierlayer is formed by oxidizing the surface of the first semiconductor filmhaving the amorphous structure with ozone generated by radiation ofultraviolet rays in an atmosphere containing oxygen.
 16. A processaccording to claim 10, wherein the first heat treatment is conducted byradiation from one or more selected from a halogen lamp, a metal halidelamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp,or a high-pressure mercury lamp.
 17. A process according to claim 10,wherein the first heat treatment is conducted by furnace annealing usingan electrically heating furnace.
 18. A process according to claim 10,wherein the second heat treatment is conducted by radiation from one ormore selected from a halogen lamp, a metal halide lamp, a xenon arclamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressuremercury lamp.
 19. A process according to claim 10, wherein the secondheat treatment is conducted by furnace annealing using an electricallyheating furnace.
 20. A process according to claim 10, wherein the raregas is one or more selected from He, Ne, Ar, Kr and Xe.
 21. A processaccording to claim 10, wherein the catalyst element is one or moreselected from Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
 22. Aprocess for producing a photoelectric conversion device, comprising thesteps of: forming a first semiconductor film having an amorphousstructure; adding an element for promoting crystallization to thesemiconductor film having the amorphous structure; conducting a firstheat treatment to form a first semiconductor film having a crystalstructure; forming a second semiconductor film over the firstsemiconductor film having the crystal structure; adding a rare gaselement to the second semiconductor film; conducting a second heattreatment to segregate the element for promoting crystallization intothe second semiconductor film, and removing the second semiconductorfilm.
 23. A process according to claim 22, wherein the rare gas is addedby ion implantation or ion doping.
 24. A process according to claim 22,wherein the first heat treatment is conducted by radiation from one ormore selected from a halogen lamp, a metal halide lamp, a xenon arclamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressuremercury lamp.
 25. A process according to claim 22, wherein the firstheat treatment is conducted by furnace annealing using an electricallyheating furnace.
 26. A process according to claim 22, wherein the secondheat treatment is conducted by radiation from one or more selected froma halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. 27.A process according to claim 22, wherein the second heat treatment isconducted by furnace annealing using an electrically heating furnace.28. A process according to claim 22, wherein the rare gas is one or moreselected from He, Ne, Ar, Kr and Xe.
 29. A process according to claim22, wherein the catalyst element is one or more selected from Fe, Ni,Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
 30. A process for producing aphotoelectric conversion device, comprising the steps of: forming afirst semiconductor film having an amorphous structure; adding anelement for promoting crystallization to the semiconductor film havingthe amorphous structure; conducting a first heat treatment to form afirst semiconductor having a crystal structure; forming a barrier layerover a surface of the first semiconductor film having the amorphousstructure; forming a second semiconductor film over the barrier layer;adding a rare gas element to the second semiconductor film; conducting asecond heat treatment to segregate the element for promotingcrystallization into the second semiconductor film; and removing thesecond semiconductor film.
 31. A process according to claim 30, whereinthe rare gas is added by ion implantation or ion doping.
 32. A processaccording to claim 30, wherein the barrier layer comprises a chemicaloxide film formed using ozone water.
 33. A process according to claim30, wherein the barrier layer is formed by oxidizing the surface of thefirst semiconductor film having the amorphous structure by plasmatreatment.
 34. A process according to claim 30, wherein the barrierlayer is formed by oxidizing the surface of the first semiconductor filmhaving the amorphous structure with ozone generated by radiation ofultraviolet rays in an atmosphere containing oxygen.
 35. A processaccording to claim 30, wherein the first heat treatment is conducted byradiation from one or more selected from a halogen lamp, a metal halidelamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp,or a high-pressure mercury lamp.
 36. A process according to claim 30,wherein the first heat treatment is conducted by furnace annealing usingan electrically heating furnace.
 37. A process according to claim 30,wherein the second heat treatment is conducted by radiation from one ormore selected from a halogen lamp, a metal halide lamp, a xenon arclamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressuremercury lamp.
 38. A process according to claim 30, wherein the secondheat treatment is conducted by furnace annealing using an electricallyheating furnace.
 39. A process according to claim 30, wherein the raregas is one or more selected from He, Ne, Ar, Kr and Xe.
 40. A processaccording to claim 30, wherein the catalyst element is one or moreselected from Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.